Caption: Caterpillar spiracles. Light micrograph of part of the skin of a caterpillar showing part of the tracheal system for bringing oxygen to the body tissues. The two spiracles (ovals, left and right) are connected by a wide tracheal tube. From each spiracle, other tracheae radiate out and branch to the surrounding tissue. The spiracles have muscles to open and close them to prevent water loss. The tracheae, which are supported by chitinous rings, branch into finer and finer tubes and allow oxygen to diffuse to the tissues, and carbon dioxide to diffuse away from the tissues. Magnification: x36 when printed at 10 centimetres across.